A two-stroke engine of this kind is disclosed in international patent publication WO98/17901 and includes a combustion chamber defined by a cylinder and delimited by a reciprocating piston. The crankcase is connected to the combustion chamber via a transfer channel. The first end of the transfer channel faces toward the cylinder and opens into the combustion chamber via an entry window lying in the cylinder wall and the lower second end of the transfer channel opens to the crankcase. The entry window of the transfer channel, which lies in the cylinder wall, is controlled by the piston in the manner of a slot control, that is, the entry window is opened or closed in dependence upon the stroke position of the piston.
The air/fuel mixture, which is necessary to operate the engine, is drawn in by suction through a mixture-preparation device and an inlet into the crankcase and, with a downward travel of the piston, is pushed into the combustion chamber via the transfer channels. The transfer channels advantageously lie opposite each other with respect to the cylinder axis. To reduce the exhaust-gas emissions, a fuel-free gas, especially air, is provided in the transfer channels and is supplied via a gas channel to each transfer channel. For this purpose, each transfer channel, between its ends, communicates with an air-conducting gas channel and a check valve configured as a membrane valve.
During the induction stroke, an air/fuel mixture is, on the one hand, inducted into the crankcase via the inlet from the mixture-preparation device when there is an upward travel of the piston in the direction of top dead center. On the other hand, fuel-free air flows in from the gas channel via the transfer channels. For a piston traveling downward in the direction of bottom dead center, the air/fuel mixture is displaced from the crankcase via the transfer channels into the combustion chamber. Because the transfer channels are filled with air, fuel-free air first flows into the combustion chamber ahead of the air/fuel mixture whereby the scavenging losses are reduced. In the following upward stroke, residual amounts of the air/fuel mixture are in the transfer channel from the previous stroke. Because of the position of the check valve between the ends of the transfer channel, a region results in the channel section to the entry window into the combustion chamber which is not scavenged by the air flowing in during the induction stroke. In a following stroke, the residual amounts of the air/fuel mixture therefore flow out of the transfer channels first and only thereafter the air, which is introduced into the transfer channel, and then the air/fuel mixture from the crankcase. The residual portions of the air/fuel mixture, which remain in the transfer channels because of the scavenging dead volume, deteriorate the exhaust-gas emissions.